Rebekah Gregory walked upright past the man accused of blowing off her leg.

The 27-year-old now uses a prosthetic limb after her own was torn off at the finish line of the Boston Marathon nearly two years ago.

On the first day of the long-awaited bombing trial, she strode past Dzhokhar Tsarnaev as he sat at the defence table and faced him from the witness box as she described the horror of his alleged crimes.

"I tried to hold it together," she told The Telegraph afterwards. "I didn't want to cry in front of him."

Ms Gregory told the jury how she was with her boyfriend and five-year-old son Noah when the first bomb went off at the marathon finish line.

The explosive, planted by Tsarnaev's elder brother, ripped through her left leg and sprayed her body with shrapnel. A brief video showed bones sticking out of a gaping wound just above her left ankle.

"I thought: 'this is the day I'm going to die,'" Ms Gregory said. Despite being deafened by the force of the blast, she said she could hear her son screaming for her. "I said: 'God if this is it take me - but let me know that my baby is ok.'"

Video footage showed Noah holding up the five fingers of his bloody hand to try to show a police officer how old he was. The little boy survived his injuries but still has a bald spot on the back of his head from a shrapnel injury.

Ms Gregory was rushed into an ambulance where a paramedic told her bluntly "this is really bad" and that her foot would have to be amputated.

"I knew at that point that if I didn’t die my legs would be gone," she said.

Ms Gregory had 18 surgeries on her injured leg, the last of which was to have it amputated, and spent a week in a coma after the bombing.

Nearly two years after the attack, she still has pieces of shrapnel in her body and will need periodic operations to remove them. "It’s just kind of one of those things,” she said, adding that she recently began using an Oscar Pistorius-style blade to run again.

Tsarnaev sat facing Ms Gregory between his two lawyers, wearing a white shirt, dark suit and no handcuffs. He showed no obvious emotion as she described the bombing.